Atomic Mass. by CHED on January 11, lesson duration of 39 minutes. under General Chemistry 1. generated on January 11, 2017 at 05:47 am

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1 Atomic Mass by CHED on January 11, 2017 lesson duration of 39 minutes under General Chemistry 1 generated on January 11, 2017 at 05:47 am Tags: Atomic, Mass

2 Generated: Jan 11, :47 PM Atomic Mass ( 2 hours and 39 mins ) Written By: CHED on July 15, 2016 Subjects: General Chemistry 1 Tags: Atomic, Mass Resources Chemistry: An atoms first approach. Zumdahl, SS. & Zumdahl, S.A. (2012). Chemistry: An atoms first approach. Belmont, CA: Brooks/Cole Cengage Learning. Chemistry: The molecular science (4th ed.) Moore, J.W., Stanitski, C.L. &Jurs, P.C. (2012). Chemistry: The molecular science (4th ed.). Belmont, CA: Brooks Cole/Cengage Learning. Chemistry. (12th ed.) Chang, R. & Goldsby, K. (2016). Chemistry. (12 th ed.). New York: McGraw-Hill. Chemistry: Atoms first Burdge, J & Overby, J. (2012). Chemistry: Atoms first. New York: McGraw-Hill. Isotopes and atomic mass [Simulation]. Retrieved from Phet Interactive Simulations website Content Standard The learners demonstrate an understanding of: 1. the mole concept in relation to Avogadro s number and mass Performance Standard The learners design using multimedia, demonstrations, or models, a representation or simulation of any of the following: 1. atomic structure 2. gas behavior 3. mass relationships in; 1 / 6

3 4. reactions Learning Competencies The learners explain relative atomic mass and average atomic mass Introduction 5 mins 1. Introduce the following learning objectives using any of the suggested protocol (Verbatim, Own Words, or Read-aloud): 1. Define atomic mass unit 2. Calculate the average atomic mass of elements 3. Determine the average molecular mass of molecules 4. Determine the average formula mass of ionic compounds 2. Present the keywords for the concepts to be learned: 1. Atomic mass unit (amu) 2. Average atomic mass 3. Molecular mass 4. Formula mass 5. Avogadro s number 6. Mole 3. Review isotopes 1. What are isotopes? 2. Give examples of isotopes. 3. What is the similarity between Mg-24 and Mg-25? What is their difference? Can you count objects by weighing them? 10 mins Ask the learners if they can count objects by weighing them. Present to them this situation: Ms. Lilia sells shelled peanuts in a store. But she meets customers asking for 150 peanuts, another for 750 peanuts, and another for 2,000 peanuts. Obviously, it will take Ms. Lilia a very long time to count the peanuts. What would be another way to count them? Ms. Lilia takes 20 peanuts and weighs them. She finds out that 20 peanuts weigh 32 g. How much then will each peanut weigh? Hence the weight of 150 peanuts would be It will be easier to weigh the peanuts than to count them. 2 / 6

4 Now, 960 g is appropriately how many peanuts? Ask them to draw a conclusion. Is it possible to count objects by weighing? Summarize the procedure done with the peanuts. This can be done with other objects like mongo beans, marbles, etc. Instruction/Delivery 35 mins I. What is the Atomic Mass and the Atomic Mass Unit? Relate the exercise on counting peanuts by weighing to counting atoms. Ask them if it is possible to use the same procedure to count atoms. Why or why not? Whether it is peanuts or mongo beans or candies or atoms, the procedure should be the same. The problem, however, is atoms are very, very small and it is not possible to see them and count them individually to get the average mass. We need to look for another way to get the average mass of the atom. Experiments have shown that atoms have different masses relative to one another. For example, a Mg atom is experimentally reported to be twice as heavy as a carbon atom; a silicon atom is twice the mass of a nitrogen atom. It is possible to make a relative scale if one atom is chosen as the reference or standard atom against which the masses of the other atoms are measured. By international agreement, the reference atom chosen is the C-12 isotope which contains six protons and six neutrons. By definition, one atom of C-12 has a mass of exactly12 12atomic mass units (amu). One amu, therefore, is one-twelfth (1/12) the mass of a C-12 atom. Figure 1: The relative scale The atomic mass of Cu-63 is amu. This means that relative to C-12, one atom of Cu-63 is 62.93/12 or times the mass of a C-12 atom. Ask them to answer this example: 3 / 6

5 One atom of Se-77 is times as heavy as an atom of C-12. What is the atomic mass of Se-77? II. Average Atomic Mass Now, ask them to look up the atomic mass for carbon in the periodic table. The expected answer is 12.01amu. Then, proceed to explaining the average atomic mass. If C has six protons and six neutrons, why is the relative atomic mass of carbon given as amu and not 12 amu? There are no individual atoms of carbon with a mass of amu. The periodic table provides the average atomic mass which takes into account the different isotopes of an element and their relative abundances. It is not a simple average that is taken but a weighted average. Illustrate a weighted average using final grade calculation: For the class in Chem345, the teacher informs the class that the final grade will be based on Exam 1 (15%), Exam 2 (15%), Problem Sets (30%), and Final Exam (40%). To pass the course, the learner must get a final grade of 75% or higher. Calculate the final grade of learner Ms. Julita if she got the following scores: Components of final grade Weight Scores of Ms. Julita Exam % 83% Exam % 95% Problem Sets 30.0% 65% Final Exam 40.0% 88% The final grade will be computed as follows: (.150 x.83) + (.150 x.95%) + (.300 x.65) + (.400 x.88) = 81% Therefore, Ms. Julita passes the course! Isotopes of elements occur in different abundances. Some are more abundant than others. Chlorine has two isotopes. The natural abundance of Cl-35 is 75% while that of Cl-37 is 25%. This means that if you have 100 atoms of chlorine, 75 of them will be Cl-35 and 25 of them will be Cl-37. Magnesium, on the other hand, has three isotopes with varying abundances: Mg-24, Mg-25, and Mg-26, have 78.99%, 10.00%, and 11.01% abundance, respectively. For carbon, the natural abundance of C-12 is 98.90% while that of C-13 is 1.10%. The atomic mass of C-13 has been determined to be amu while that of C-12 is exactly 12 amu. Now, we calculate the average atomic mass of carbon: = (atomic mass of C-12) (% abundance of C-12) + (atomic mass of C-13) (% abundance of C-13) = ( amu) (.9890) + ( amu) (.0110) = amu Ask them to answer these practice exercises: 1. From the periodic table, look up the average atomic mass of the following elements: Co, Be, Al, Zn. 2. Copper has two stable isotopes with the following masses and % abundances: Cu-63 (62.93 amu, 69.09% abundance) and Cu-65 ( amu, 30.91% abundance). Calculate the average atomic mass of copper. 3. An element consists of an isotope with mass of amu and 19.91% abundance, and another isotope 4 / 6

6 with mass of amu and 80.09% abundance. Calculate the average atomic mass of this element. Refer to the periodic table and identify the element. III. Average Molecular Mass (also referred to as molecular mass) The molecular mass is the sum of the average atomic masses of the atoms in the molecule. Ask them to answer the following examples: 1. What is the molecular mass of carbon dioxide, CO 2? 2. Determine the molecular mass of the following molecules: 1. Water, H 2 O 2. Methane, CH 4 IV. Average Formula Mass (also referred to as formula mass) The formula mass is the sum of the atomic masses of the atoms in the ionic compound. Ask them to answer the following examples: 1. What is the formula mass of sodium chloride, NaCl? 2. What is the formula mass of magnesium chloride, MgCl 2? Vitamins and Minerals 99 mins 1. Vitamins and minerals are nutrients for the body. An example of a vitamin is Vitamin C. Look up the molecular formula of Vitamin C and determine its average molecular mass. What is another common name for Vitamin C? Give at least one important use of Vitamin C in the body. 2. Minerals include potassium, calcium, iron, and zinc. Look up the average atomic mass of calcium, Ca. Give at least one important use of Ca in the body. Check-up Quiz 10 mins Answer the following questions. Place the answers in the space provided. Show calculations where applicable. Observe the use of significant figures for calculations and indicate the appropriate units. Learners can use the periodic table to answer the questions. 1. From the periodic table, look up the average atomic mass of bromine, Br. Answer key 2. How much heavier is an atom of Br relative to an atom of carbon? 3. Which element in the periodic table has an average atomic mass that is about ten times that of fluorine? Element A consists of isotope A-6 with natural abundance of 7.5% and a mass of amu, and isotope A-7 with amu times heavier Osmium, Os 5 / 6

7 Powered by by TCPDF ( CHED.GOV.PH natural abundance 92.5% and mass of amu. 4. Calculate the average atomic mass of element A. 5. Identify Element A. Naphthalene has the molecular formula C 8 H How many elements make up one molecule of naphthalene? What are they? 7. What is the molecular mass of naphthalene? 6.94 amu Lithium, Li Two elements; Carbon and hydrogen amu amu 8. What is the formula mass of potassium chloride, KCl? Generated: Jan 11, :47 PM 6 / 6